Two soils amended with a sewage sludge were incubated at two ambient temperatures (15 degrees C and 25 degrees C) under greenhouse conditions for two years. Representative samples of the sewage sludge and periodically collected soil samples were subjected to an operationally-defined sequential extraction procedure. This procedure fractionated metals in three broad fractions through the use of CH3COOH, NH2OH . HCl and H2O2 in sequence. In the sludge, the greatest percentages (63-98%) of the five metals studied (Cd, Cu, Ni, Pb and Zn) were extracted by H2O2/NH4OAc. The application of sludge td soils substantially increased the contents of metals in all the three fractions. Cadmium, Ni and Zn in the sludge-amended soils were predominantly in the CH3COOH extractable fraction while Cu and ph were extracted primarily by H2O2/NH4OAc. The contents of Cd, Cu, Pb and Zn extractable by CH3COOH increased in samples collected over a period of two years while that of Ni did not show any significant change. Metal concentrations extractable by NH2OH . HCl remained more or less constant over the residual period. Metal contents extracted in H2O2/NH4OAc decreased considerably during the first year before stabilising for the rest of the study period. The different incubation temperatures had no obvious effect on CH3COOH and NH(N)2OH . HCl extractable metal fractions. However, the observed decline in metal contents, over time, extracted by H2O2/NH4OAc was faster in samples collected from the warm environment (25 degrees C) than those from cooler conditions (15 degrees C). The observed trend of increasing metal concentrations in CH3COOH- extractable fraction and decrease in the H2O2/NH4OAc fraction coincided with decreasing soil organic matter status due to microbial oxidation of sludge-added organic matter. The importance of trace metal fractionation and transformation between fractions are discussed.
